1. Field of the Invention
The present invention relates to an electrophotographic or electrostatic image forming apparatus such as a copying machine or a printer.
2. Description of the Related Art
An image forming apparatus that forms a toner image on a recording material has a fixing unit for heat-fixing the toner image to the recording material. In the case where a toner image formed on a narrow recording material is fixed by the fixing unit, non-paper-passage regions through which the recording material does not pass overheat, and so various countermeasures to this phenomenon have been proposed. As one of them, Japanese Patent Laid-Open No. 2008-52031 discloses blowing on the non-paper-passage regions of the fixing unit and thereby cooling the non-paper-passage regions. In a low heat capacity fixing unit such as a fixing unit of film heating type including an endless belt (fixing film), the heater does not generate heat during standby while waiting for a print signal. Even if the heater generates heat, the fixing unit is warmed to a very low temperature. Even in such a standby state, the fixing unit can be brought to a fixable state in a short time after the reception of a print signal. On the other hand, because the fixing unit is not warmed during standby, the pressure roller is likely to cool. For this reason, in the case where a recording material containing a large amount of moisture undergoes a fixing process with the temperature of the pressure roller low as in the early stage of printing (for example, the first sheet of continuous printing), the moisture contained in the recording material evaporates at once in the fixing nip portion and is likely to condense on the surface of the pressure roller. This condensation reduces the friction coefficient of the pressure roller and may cause a phenomenon in which the recording material slips and is not conveyed. This slip of the recording material will be referred to as condensation slip. In a fixing device of film heating type, a fixing film is driven by the rotation of a pressure roller, and a recording material is conveyed only by the driving force of the pressure roller. For this reason, if condensation occurs on the pressure roller, the conveying force decreases rapidly and the above-described slip phenomenon is likely to occur.
In order to solve this problem, it is possible to use fans for preventing excessive temperature rise of the non-paper-passage portions, to diffuse water vapor generated around the fixing unit with air taken in by the blower fans from the outside of the image forming apparatus, and to prevent condensation on the pressure roller. However, if air from the blower fans is sent to the fixing unit with the temperature of the pressure roller low, the temperature of members such as the pressure roller and the fixing film decreases, the fixing property of the toner image decreases, and an image defect may be caused.